Underwriters calculate two ratios from the same income, and the second one turns down more applications than credit score does.
The two ratios
Front-end (housing) ratio — the proposed housing payment divided by gross monthly income. Housing means principal, interest, property taxes, homeowner's insurance, and any HOA dues or mortgage insurance. The usual guide is 28%.
Back-end (total) ratio — every monthly debt obligation, including that housing payment, divided by the same gross income. Conventional loans commonly cap at 43–45%, sometimes 50% with strong compensating factors. FHA can stretch further with good reserves.
Gross means before tax. The ratios are calculated on income you have not received yet, which is one reason a "qualified" payment can feel unaffordable in practice.
What counts as debt
Counted:
- credit card minimum payments, not the balance
- car loans and leases
- student loans, even when deferred — a percentage of the balance is generally imputed
- personal loans
- child support and alimony paid
- any other loan reported to the bureaux
Not counted:
- utilities, phone, internet, streaming
- groceries, fuel, childcare
- insurance other than housing-related
- taxes withheld from your pay
That list is worth reading twice. Childcare at $1,800 a month does not appear in the calculation at all, while a $300 car payment does. The ratios measure lender risk, not household budget, and the gap between the two is where people get into trouble with a payment they technically qualified for.
A worked example
Gross income $8,000 a month. Existing debts: $450 car, $120 card minimums, $200 student loan — $770.
At a 45% back-end cap, total obligations may reach $3,600. Subtract $770 and $2,830 is available for housing.
That $2,830 has to cover taxes and insurance as well as the loan. If taxes and insurance come to $700, the principal-and-interest budget is $2,130 — which at 6.5% over 30 years supports a loan of roughly $337,000.
Why paying off a car can beat improving your score
Take the same borrower and clear the $450 car loan. Available housing payment rises to $3,280; principal and interest to $2,580; supported loan to roughly $408,000.
A $450 monthly payment was worth about $71,000 of purchase power. Fifty points of credit score, by comparison, might move the rate a quarter point — around $60 a month, or roughly $10,000 of purchase power.
The order of operations follows directly. If you are within a few months of applying and have spare cash, retiring a small instalment loan usually does more than anything you can do to the score.
Do not pay a card down to zero and close it
Paying a card down helps utilisation and therefore the score. Closing it removes the limit from the utilisation calculation and can raise your ratio on the remaining cards immediately.
Paying off a card also does not reduce the back-end ratio much, because only the minimum payment counted — perhaps $25. Paying off a car removes the whole payment. That is why instalment loans are the better target.
Income the underwriter will not use
Not everything that arrives in your account counts:
- Self-employment income generally needs two years of tax returns, and it is the net figure after deductions that is used. Deducting aggressively for tax reasons reduces borrowing power directly.
- Bonus and commission usually need a two-year history, and are averaged.
- A new job in the same field is normally fine; a career change often resets the clock.
- Rental income is typically counted at about 75%, to allow for vacancy and maintenance.
Overtime that has run for eighteen months and stopped, or a bonus received once, will not be included however real it felt.
The gap between qualifying and affording
Nothing in the calculation accounts for childcare, medical costs, how much you save, or what you spend. A 45% back-end ratio on gross income can be well over 60% of take-home once tax is deducted.
Lenders are measuring the likelihood of default, and that is a lower bar than the payment being comfortable. It is entirely reasonable — and common — to borrow meaningfully less than the maximum offered.